Your natural stone countertops will be sealed at the time of installation. It is highly recommended that you keep your countertops sealed by applying fresh sealer annually with a good quality penetrating sealer. The counters would be well cleaned and fully dry before starting the sealing process. There are several good sealers on the market. You can find impregnating sealers at Lowe's and Home Depot, as well as the stone suppliers where slabs are purchased. Instructions are on these products, and are easy to follow.
Mild soap and water are always the best thing to use on "natural" stone countertops. Never use anything acidic (Windex, ammonia, vinegar, or citrus) or hard abrasive cleaners that could etch or dull the surface. Oven cleaners are especially damaging. A good granite and marble polish or natural bee's wax is something you will want to purchase and use weekly.
Do's and Don'ts
Granite, marble and quartzite are all "natural" stones and must be treated with care. Do not stand or sit on the counters. Overhangs of more than 6 inches will need to have supports. Heat will not hurt the stone and, with granite, knives will not scratch the surface (but the granite will dull your knives). Do enjoy your new BEAUTIFUL stone countertops. No other surface can compare with the beauty of Natural Stone Countertops.